Abstract

Many organisms have developed defences
to avoid predation by species at higher trophic
levels. The capability of primary producers to defend
themselves against herbivores affects their own survival,
can modulate the strength of trophic cascades
and changes rates of competitive exclusion in
aquatic communities. Algal species are highly flexible
in their morphology, growth form, biochemical
composition and production of toxic and deterrent
compounds. Several of these variable traits in
phytoplankton have been interpreted as defence
mechanisms against grazing. Zooplankton feed with
differing success on various phytoplankton species,
depending primarily on size, shape, cell wall structure
and the production of toxins and deterrents.
Chemical cues associated with (i) mechanical damage,
(ii) herbivore presence and (iii) grazing are the
main factors triggering induced defences in both
marine and freshwater phytoplankton, but most
studies have failed to disentangle the exact mechanism(
s) governing defence induction in any particular
species. Induced defences in phytoplankton
include changes in morphology (e.g. the formation
of spines, colonies and thicker cell walls), biochemistry
(such as production of toxins, repellents) and
in life history characteristics (formation of cysts,
reduced recruitment rate). Our categorization of
inducible defences in terms of the responsible
induction mechanism provides guidance for future
work, as hardly any of the available studies on
marine or freshwater plankton have performed all
the treatments that are required to pinpoint the actual
cue(s) for induction. We discuss the ecology of
inducible defences in marine and freshwater phytoplankton
with a special focus on the mechanisms of
induction, the types of defences, their costs and
benefits, and their consequences at the community
level.